1. Field of the Invention
This invention relates to power converters and, in particular, to autonomous inverters.
2. Description of the Prior Art
At present, adjustable electric drives equipped with autonomous inverters are extensively used in many technical fields. They substantially simplify the kinematics of machines and mechanisms, eliminate reduction gears, combine the electric drive and the mechanism into an integrated device, make the technilogical process fully automatic, and maintain desired optimal conditions of the mechanism with high speed and accuracy.
The circuitry of an autonomous inverter, which is one of the basic components of a thyristor frequency changer, largely determines the adjusting characteristics and power performance of the electric drive, as well as its weight, size, cost, and reliability. The latter considerations often dictate the choice of a particular circuit in preference to others, when selecting an autonoumous inverter for a high-power, high-voltage frequency converter. Many autonomous inverters are rejected for purely practical reasons.
In this connection it is a serious problem to provide autonomous inverters which are structurally uncomplicated, functionally reliable to provide improved communtation stability, and have better power performance. They should be smaller in size, lighter in weight, and less costly to manufacture. The problem is especially applicable to autonomous inverters for high-voltage frequency-adjustable electric drives wherein addition of extra gates and capacitors is impractical.
Known in the art is an autonomous inverter (cf., for example, USSR Inventor's Cetrificate No. 505,099 Cl. H02 m 5/42, H02 m 7/472) which is designed for induction motor control. In order to improve commutation stability and ensure faultless starting, this autonomous inverter, comprising thyristor switches, a differential-transformer control transducer whose primary winding is inserted into the circuit composed of a capacitor and one of the thyristor switches, is provided with additional threshold limiters whose inputs are connected to the transducer secondary winding. Outputs of the limiters are connected to inputs of a step phase shifter, the input of the master oscillator is connected to output lines of the autonomous inverter, while the output thereof is connected to the input of the step phase shifter.
The threshold limiters are adjusted to one level of the signal of the differential-transformer transducer in order to maintain the amplitude of the voltage across the switching capacitors constant in all conditions of the autonomous inverter and improve its commutation stability.
This autonomous inverter is deficient in that the switching capacitors are not isolated from the load during the intervals between switchings, and energy-exchange oscillating processes of persistent nature exist between the leakage induction motor of the switching capacitors. This results in distortion of the shape of the output voltage of the autonomous inverter by harmonics. Moreover, a direct component may appear on the switching capacitors of this autonomous inverter, which results in the asymmetry of the restoration angles of the switch thyristors and, consequently, in worse commutation stability. Any control assymetry leads to the voltage direct component appearing on the capacitors due to the presence of the transformer in the control transducer, through which information on the direct component cannot be transmitted.
Also known in the art is an autonomous inverter (cf., for example, W. Echner, D. Kollensperger, "Einrichtung zur Zwangskommutierung fur eine Anordnung zur Steuerung der Drehzahl und Drehrichtung einer Drehstrommaschine", Siemens AG, FRG Patent No. 1638551, Cl. H02P 7/62, filed Feb. 17, 1967, published May 28, 1975) comprising a main bridge thyristor switch, switching capacitors, and an additional thyristor bridge switch.
In order to improve commutation stability, each switching capacitor is provided with a charging device comprising a transformer, a choke, capacitors, and thyristors. The inverter possesses good commutation stability due to the reliable recharging of the switching capacitors by the beginning of each next switching period. Since the commutating capacitors are isolated, in each switching interval, from the load (induction motor), no serious distortion of the shape of the output voltage by harmonics takes place. But this autonomous inverter has the disadvantage that the voltages applied to the thyristors of the additional bridge switch, during current commutation from one phase of the induction motor to another, are twice as large as those applied to the thyristors of the main thyristor bridge switch. To cope with this problem, thyristors of the additional switch should be rated for higher voltage or series-connected thyristors should be used. Both solutions make the inverter more expensive, heavier, and larger, particularly in high-voltage applications.
This autonomous inverter is not sufficiently reliable because it is composed of a large number of components which are functionally connected in a sequential arrangement.
Also known in the art is an autonomous inverter (cf., for example, USSR Inventor's Cetrificate No. 811,460 Cl.H02 m 7/515) comprising two commutating thyristors and an m-phase thyristor bridge whose a.c. leads are connected, via commutating capacitors, to respective a.c. leads of an auxiliary m-phase diode bridge, while d.c. leads of the diode bridge are connected with the like d.c. leads of the thyristor bridge via the commutating thyristors coupled with diodes in an aiding connection. Thyristors of the autonomous inverteer are controlled by a master oscillator whose output is connected to an input of a distribution unit which distributes control pulses to respective thyristors. A damping resistor is connected to the d.c. leads of the auxiliary diode bridge, which are coupled with the like d.c. leads of the thyristor bridge via the commutating thyristors which are in aiding connection with the diodes, in order to improve the commutation stability of the autonomous inverter and reduce the commutation surges on the inverter load. The commutation stability is improved, and voltage surges are limited due to damping the oscillating processes during current commutation. This is achieved by selecting resistors which have a resistance equal to 0.6-0.8 of the characteristic impedance of the circuit comprising a commutating capacitor and load phase inductors.
But the autonomous inverter is deficient in that its efficiency is affected by the losses in the damping resistor selected in accordance with the conditions described above. The losses can be substantially reduced when the resistance of the damping resistor is sufficiently high, the commutating capacitors can still be disconnected from the load in the intervals between the switching. But the voltage amplitude on the commutating capacitors cannot be stabilized by any known circuit arrangements, and this results in the appearance of an uncontrollable d.c. voltage component on the capacitors. The commutating capacitors, thyristors, and diodes are to be rated for higher voltage. This makes the equipment much more expensive, heavier, and bulky.